For example, I've read one of the reasons South Africa pursued a nuclear weapons program was to stave off an invasion from other countries for upholding apartheid, so when they ended apartheid, they likewise gave up nuclear weapons, the only nation known to have done that.
Bearing in mind that this may come off as overly jingoistic, or extremely pro-America, but the question is phrased such that it's mostly inevitable.
I'll start with the flawed assumption that the idea of humanitarian rights and business interests are necessarily apart in the US's eyes. It often is the case with the US that democracies do business with the US not because the US forces them to, but because once they're open to the concept of business and the free market, doing business with the US is profitable.
So, this means that the US can invade a country for purposes of opening up the country to business, ("Bananas" by Peter Chapman). This, in the turn of the century period especially, led the US to what are called "Banana wars" (highly recommend the Wikipedia article on them! That's not to say that no humanitarian purposes came of them, but it's fair to say that the primary purpose was to open certain countries, like Nicaragua, which the US bombarded and had troops occupy at the urging of US business interests. They did this claiming to be defending American citizens and American property, which is pretty transparent in my opinion (this article delightfully implies that critical detail in the throwaway line "American women have been taken aboard the American gunboats for safety").
But this also means that the US could invade a country for purposes of humanitarian interests, while also coincidentally opening it up to US business interests. For example, our intervention in Grenada likely resulted in better American business opportunities, due to our increased relations with the country after the intervention, but it's also safe to say that it was for primarily humanitarian purposes. The New Jewel movement captured political prisoners and the legitimate leaders of the country asked the US for help. The US intervened, basically restored the legitimate leadsership of the country, then left. Grenadans still celebrate "Thanksgiving" on 25 October, commemorating and thanking the US for their invasion in 1983.
So your question, "Has the US ever actually engaged in an invasion for strictly humanitarian purposes" is a bit... I dunno not really fair? It's hard to say that anything is ever strict in the sense of history. There are always dozens of reasons contributing to anything. But that being said, I think you could fairly say that at least in that particular case, the effort was at least overwhelmingly humanitarian and resulted in tremendous positives for the population of the country, and isn't talked about.
But honestly, why stop there? see my reply to this comment.